Tibetan people inherited a genetic variant from an extinct subspecies
of humans that helps them survive on the 'roof of the world'.

Life is hard at high altitudes. On the Tibetan plateau, over
4,000 metres above sea level, there is less oxygen in the air, the climate is
cold and vegetation sparse. The body responds to low levels of oxygen by
producing extra haemoglobin, the molecule that carries oxygen in red blood
cells. However, this thickens the blood, raising blood pressure and increasing
the risk of stroke as well as complications during pregnancy.

The gene in question, EPAS1, helps protect against these
risks by limiting the number of red blood cells produced. The Tibetan version
of EPAS1 is different to that found in other populations. It occurs in 87
percent of Tibetans compared to nine percent of Han Chinese - the largest
ethnic group in China.

Professor Rasmus Nielsen from the University of
California, Berkley, USA, who co-authored the paper, said the study provided 'very clear evidence that this version of the gene came from Denisovans', an
ancient group of humans who lived in Asia around 41,000 years ago.

The researchers sequenced the EPAS1 gene in 40 Tibetan and
40 Han people. The DNA code for the Tibetan EPAS1 looked so different that it
seemed unlikely to have evolved during the 3,000 years since the group
separated from Han people. After searching through other genomes, the team
discovered that this unique copy of EPAS1 probably got into Tibetans through crossbreeding
with Denisovans.

Everything we know about Denisovans comes from a tiny part
of a finger bone, two teeth and a toe, discovered four years ago in a
Siberian cave. Scientists were able to sequence DNA from these bones and piece
together a Denisovan genome.

This study, published in Nature, adds weight to the idea
that interbreeding with ancient human species, most famously Neanderthals,
significantly affected the course of human evolution.

Professor Svante Pääbo, a geneticist at the Max Planck
Institute for Evolutionary Anthropology in Leipzig, Germany, who originally sequenced
the Denisovan genome, told New Scientist: 'It is very satisfying to see that
gene flow from Denisovans [...] is now found to have had important consequences
for people living today'.